Method of forming multi-layer paint films, base coat composition and method of painting

ABSTRACT

To form multi-layer paint films which have a good finished appearance on objects which are to be painted, using an aqueous base coat composition. An aqueous base coat composition which contains aqueous polyurethane resin and rheology control agent, the electrical conductivity measured in an aqueous solution of 3% by weight concentration of which is 700-900 mS/cm, is painted and then the paint film is formed, a transparent top coat is painted on the base coat paint film obtained in this way, and the base coat and the top coat are baked at the same time.

INDUSTRIAL FIELD OF APPLICATION

This invention concerns a method of forming multi-layer paint films,base coat compositions and a method of painting and, more precisely, itconcerns a method of forming multi-layer paint films with which a goodfinished appearance can be obtained, aqueous base coat compositionswhich have excellent storage stability and with which paint films whichhave a good finished appearance can be obtained, and a method ofpainting therewith.

PRIOR ART

In general, a series of paints are painted onto the base when paintingautomobiles. The first coat is an electrodeposited primer and, afterthis, a mid-coat paint is painted on and then the base coat for theovercoat paint is painted on over this and finally a clear coat ispainted on as a top coat. The base coat is the one which imparts gooddecoration to the final finish by means of a organic pigment orinorganic pigment, and a metallic finish is desirable for the finish ofmany automobiles. A metallic pigment, such as aluminium flakes forexample, is generally compounded in the base coat in order to obtain themetallic effect.

Paint film formation of the base coat and the clear coat (top coat) isnormally achieved by wet-on-wet painting, which is to say that the clearcoat is painted on without baking after painting on the base coat andthen flashing off (drying) for a short period of time at roomtemperature, and subsequently baking and drying the base coat and clearcoat at the same time.

Large amounts of organic solvent are included in the paints, andespecially the base coat paints, for automobile purposes which are onthe market at the present time. However, concern over the volatileorganic solvents which are released into the atmosphere is increasing,and much effort is being directed towards the research and developmentof aqueous paints which contain water in the main as the solvent and inwhich the amount of organic solvent is small.

The orientation of the metallic pigment flakes in the paint filmcontributes to the metallic effect. Hence, in order to obtain a metalliceffect with a base coat, the metal pigment flakes which are present inthe base coat must be orientated appropriately after painting. They haveto be immobilized immediately, and care must be taken to ensure thatthey are not disturbed in the next process, namely clear coat painting.

Difficult problems arise as a result of the physical properties of waterwhen developing water based paints which have a metallic effect. At thepresent time, compositions which can be used as water based base coatsare limited from the viewpoint of orientation as mentioned above. Waterbased paints which have a metallic effect have been disclosed, forexample, in European Patents EP-A-89497, EP-A-355433 and EP-A-521919,German Patents DE-OS-3545618, DE-OS-3903804 and DE-OS-4107136, and U.S.Pat. No. 4,719,132.

The use of base coats in which silica thickeners which have thixotropicproperties are compounded in multi-layer film formation is disclosed inthe abovementioned patents, and painting with a view to providing beautyand protection is carried out by forming a base coat/clear coat typemulti-layer film on the base material using base coats of this type.

However, with base coats which contain silica such as those indicatedabove, the storage stability of the paint at high temperature is poorand there is a difficulty in that a good paint film finished appearancecannot be obtained.

Furthermore, an aqueous paint composition wherein an aqueouspolyurethane resin and various silica based thickeners are combined hasbeen disclosed in the abovementioned European Patent EP-A-89497.However, in this patent there is no indication at all as to the type ofsilica based thickener which is used, or the electrical conductivity ofthe thickener.

Furthermore, the combination of sodium magnesium silicate and acryliccopolymer based thickener and/or other synthetic resin based thickenerand/or auxiliary thickener (associative thickener) has been disclosed inGerman Patent DE-A-4110520, but similarly there is no disclosureconcerning electrical conductivity.

PROBLEMS TO BE RESOLVED BY THE INVENTION

The aim of this present invention is to propose a method of formingmulti-layer paint films with which paint films which have a goodfinished appearance can be obtained using an aqueous base coatcomposition.

A further aim of the invention is to provide an aqueous base coat paintwhich has excellent storage stability and with which paint films whichhave a good finished appearance can be formed, and a method of paintingtherewith.

MEANS OF RESOLVING THESE PROBLEMS

The present invention is a method of forming multi-layer paint films, abase coat composition and a method of painting as indicated below.

(1) Method of forming multi-layer paint films wherein a multi-layerpaint film is formed on an object which is to be painted with a view toproviding protection and/or beauty, characterized in that:

1) a base coat composition which is aqueous and which contains pigmentis painted on as a base coat;

2) a paint film of the composition which has been painted on in 1) isthen formed;

3) a transparent top coat is painted on the base coat paint film whichhas been obtained in this way; and

4) the base coat and the top coat are baked at the same time.

(2) Base coat composition comprising an aqueous dispersion for forming amulti-layer paint film on an object which is to be painted with a viewto providing protection and/or beauty, characterized in that itcomprises a polyurethane dispersion wherein an aqueous polyurethaneresin which has structural units represented by the general formula (1)indicated below is dispersed in water. ##STR1## (In this formula, Yrepresents an aromatic, aliphatic or alicyclic hydrocarbyl group.)

(3) Base coat composition, according to (2) above, characterized in that1-80% by weight of aqueous melamine resin is included with respect tothe solid fraction of the polyurethane dispersion.

(4) Base coat composition, according to (3) above, characterized in thatit contains aqueous polyester resin and/or acrylic resin as a bindercomponent, the Melamine Resin!: Polyester Resin and/or Acrylic Resin!ratio by weight is 2:1-1:4, and the total amount of melamine resin,polyester resin and acrylic resin is 1-80% by weight with respect to thesolid fraction of the polyurethane dispersion.

(5) Base coat composition, according to any of (2)-(4) above,characterized in that a rheology control agent is included in an amountof 0-10% by weight, and preferably of 0.5-2% by weight, with respect tothe base coat composition.

(6) Base coat composition, according to any of (2)-(5) above,characterized in that it contains a rheology control agent whichexhibits a specified electrical conductivity.

(7) Base coat composition, according to any of (2)-(6) above,characterized in that it contains a rheology control agent of which a 3%aqueous solution exhibits an electrical conductivity of 700-900 mS/cm,and preferably of 700-780 mS/cm.

(8) Base coat composition, according to any of (2)-(7) above,characterized in that sodium magnesium silicate is included as arheology control agent.

(9) Base coat composition, according to any of (2)-(8) above,characterized in that it contains 0.5-25 by weight of metallic pigmentand/or effect pigment.

(10) Base coat composition, according to any of (2)-(9) above,characterized in that the pH value is 6-9, preferably 7.5-8.5, and mostdesirably 8.0-8.4.

(11) Method of painting a base coat composition as disclosed in any of(2)-(10) above which is used on an object which is to be painted.

(12) Method of painting a base coat composition as disclosed in any of(2)-(10) above which is used on plastic.

(13) Method of painting a base coat composition as disclosed in any of(2)-(10) above which is used for repainting purposes.

(14) Method of forming a multi-layer paint film, according to (1) above,characterized in that a base coat composition which contains 1-80% byweight of the aqueous melamine resin with respect to the solid fractionof the polyurethane dispersion is used.

(15) Method of forming a multi-layer paint film, according to (14)above, characterized in that a base cost composition which containsaqueous polyester resin and/or acrylic resin is included, of which theMelamine Resin!/ Polyester Resin and/or Acrylic Resin! ratio by weightis 2:1-1:4, and in which the total amount of melamine resin, polyesterresin and acrylic resin is 1-80% by weight with respect to the solidfraction of the polyurethane dispersion is used.

(16) Method of forming a multi-layer paint film, according to any of(1), (14) and (15) above, characterized in that a base coat compositionwhich contains a rheology control agent in an amount of 0-10% by weight,and preferably of 0.5-2% by weight, with respect to the base coatcomposition is used.

(17) Method of forming a multi-layer paint film, according to any of(1), (14) and (15) above, characterized in that a base coat compositionwhich contains a rheology control agent of which a 3% aqueous solutionexhibits an electrical conductivity of 700-900 mS/cm, and preferably of700-780 mS/cm, is used.

(18) Method of forming a multi-layer paint film, according to any of(1), (14) and (15) above, characterized in that a base coat compositionwhich contains sodium magnesium silicate as a rheology control agent isused.

(19) Base coat composition, according to any of (2)-(10) above, whereinit is used in combination with an aqueous top coat or a powder top coat.

In the method of forming a multi-layer paint film of this presentinvention, a base coat composition which is aqueous and which containspigment is first of all painted as a base coat onto the object which isto be painted. A base coat composition as disclosed in claims 2-10, and,among these, a base coat composition which contains a rheology controlagent which exhibits a specified electrical conductivity, is preferredfor the base coat composition which is used here, but it is not limitedto these compositions. A paint film of the base coat composition is thenformed by drying for a short period of time at from room temperature toabout 80° C. Next, a transparent top coat is painted on to the base coatpaint film without baking. The base coat and top coat are then baked atthe same time and a multi-layer paint film is formed.

The aqueous polyurethane resin used in the invention is a resin whichcontains structural units which can be represented by the aforementionedgeneral formula (1). Actual examples of structural units which areincluded in general formula (1) include the structural units representedby general formula (2) below. ##STR2## (In this formula, X represents anaromatic, aliphatic or alicyclic hydrocarbyl group, and R¹ -R⁴ representalkyl groups of carbon number 1-4.)

Examples of alkyl groups of carbon number 1-4 which can be representedby R¹ -R⁴ in general formula (2) include the methyl, ethyl, n-propyl,isopropyl, n-butyl, iso-butyl, sec-butyl and tert-butyl groups. R¹ -R⁴may all be the same, or some of them may be different, or all of themmay be different.

The following groups can be cited as divalent aromatic hydrocarbylgroups which are shown as Y in general formula (1). ##STR3##

The dimethylene group, the tetramethylene group and the hexamethylenegroup can be cited, for example, as aliphatic hydrocarbyl groups whichcan be represented by Y.

The groups indicated below can be cited, for example, as alicyclichydrocarbyl groups which can be represented by Y. ##STR4##

No particular limitation is imposed upon the proportion of structuralunits represented by the aforementioned general formula (1) in theaqueous polyurethane resin, but it is preferably 10-40% by weight.Furthermore, no particular limitation is imposed upon the molecularweight of the aqueous polyurethane resin, but it is preferably a numberaverage molecular weight of at least 1000, more preferably at least4000, and even more particularly preferably 5000-8000.

Salts can be formed by adding primary--tertiary organic amines, such astriethylamine, diethylethanolamine or N-methylmorpholine, or ammonia, todisperse the aqueous polyurethane resin in water, or an aqueous mediumin which water is the principal component, and obtain an aqueousdispersion. A polyurethane dispersion is obtained in this way.

Aqueous polyurethane resins, or aqueous dispersions thereof, used in theinvention have been disclosed, for example, in European PatentsEP-A-89497 and EP-A-355433, German Patents DE-OS-3545618, DE-OS-3903804and DE-OS-4107136, and U.S. Pat. No. 4,719,132.

The aqueous polyurethane resins used in the invention can bemanufactured, for example, by polymerizing the components 1)-4) asindicated below.

1) Polyester polyol or polyether polyol, or a mixture of thereof. Thoseof these compounds of number average molecular weight 400-5000 arepreferred.

2) Diisocyanate which can form structural units which can be representedby the aforementioned general formula (1) by reacting with hydroxygroups, or mixtures thereof. Actual examples include tetramethylxylenediisocyanate (m-TMXDI), 1,3-bis(2-isocyanatopropyl-2-yl)benzene),isophorone diisocyanate, hexamethylene diisocyanate,bis(isocyanatocyclohexyl)methane and 1,4-tetramethylene diisocyanate.

3) Compound which has 2 or more functional groups, such as hydroxygroups and/or amino groups, or a mixture thereof. Those of thesecompounds of number average molecular weight 60-400 are preferred.Actual examples include ethylene glycol, diethylene glycol, triethyleneglycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butyleneglycol, 1,6-hexanediol, trimethylolpropane, castor oil, hydrogenatedcastor oil, ditrimethylolpropane ether, pentaerythritol,1,2-cyclohexanediol, 1,4-cyclohexanedimethanol, bisphenol A, bisphenolF, neopentyl glycol, hydroxypivalic acid neopentyl glycol ester,hydroxyethylbisphenol A, hydroxypropylbisphenol A, hydrogenatedbisphenol A, and mixtures thereof.

4) Compound which has just 1 functional group which reacts withisocyanate group in the molecule, or mixtures thereof. Actual examplesinclude methanol, ethanol, propanol, butanol and pentanol.

A crosslinked structure is obtained when an aqueous melamine resin iscompounded as a binder in addition to the aqueous polyurethane resin ina base coat composition of this present invention, and this is desirablesince a paint film which has superior water resistance and physicalresistance is obtained. Moreover, if the acrylic resin dispersionsdisclosed in German Patent DE-OS-4009857 and/or the aqueous polyesterresins disclosed in European Patent EP-A-89497, German PatentDE-OS-3210051 or German Patent DE-OS-4107136, and/or polyether resins,are combined in addition to the aqueous melamine resin, thedispersibility of the pigment is improved and the smoothness of thepaint surface is good, and so this is desirable.

The storage stability is especially good when a rheology control agentis compounded in a base coat composition of this present invention, andthis is also desirable since it is possible to form paint films whichhave an especially good finished appearance. The rheology control agentis one of which the electrical conductivity, measured at a concentrationof 3% by weight in aqueous solution, is 700-900 mS/cm, preferably700-800 mS/cm, and most desirably 700-780 mS/cm. Here, a rheologycontrol agent is one of those known in the past as a rheology controlagent, thickener, thinner, thixotropic agent or anti-run agent forexample, which has been compounded to thicken and stabilize the paint.The rheology control agents which have been used conventionally forwater based paints can be used in this present invention. Examplesinclude inorganic materials such as silicates, montmorillonite, organicmontmorillonite and colloidal alumina; cellulose derivatives such ascarboxymethylcellulose, methylcellulose and hydroxyethylcellulose;protein based materials such as casein, sodium caseinate and ammoniumcaseinate; alginic acid based material such as sodium alginate;polyvinyl based materials such as poly(vinyl alcohol),polyvinylpyrrolidone and poly(vinyl benzyl ether) copolymers;polyacrylic acid based materials such as poly(sodium acrylate) andpoly(acrylic acid)--methacrylic acid ester copolymers; polyether basedmaterials such as pluronic polyethers, polyetherdialkyl esters,polyetherdialkyl ethers, urethane modified polyethers and epoxy modifiedpolyethers; and maleic acid anhydride copolymer systems such aspartially esterified vinyl methyl ether--maleic anhydride copolymer.From among these the silicates, and especially sodium magnesiumsilicate, are preferred. Sodium magnesium silicate is generally calledsmectite clay or synthetic hectorite, and it is marketed by Laport, forexample, under the trade name LAPONITE RD (sodium magnesium lithiumsilicate) and it is readily available commercially. Sodium magnesiumsilicate assumes a lower electrical conductivity on washing with water,even though the composition is the same, and an electrical conductivityin the abovementioned range can be achieved by controlling the extent ofwater washing.

The rheology control agent, for example sodium magnesium silicate, ispreferably used as an aqueous paste. Most desirably, it is in the formof a 3% by weight sodium magnesium silicate solution in water togetherwith 3% by weight of polypropylene glycol, a 2% by weight sodiummagnesium silicate solution in water together with 0.6% by weight ofpolypropylene glycol, or a 2% by weight sodium magnesium silicatesolution in water together with 2% by weight of other surfactant. Thepercentages by weight indicated above are values with respect to thetotal weight of the paste.

The rheology control agent, for example sodium magnesium silicate, canbe used alone, or it can be used in combination with other ionicsynthetic resin based thickeners and/or auxiliary thickeners (associatethickeners, for example OLEATE 205; trade name, manufactured by theReokkusu Co., or BISUKAKAKKU HV30; trade name, manufactured by theAllied Colloid Co.).

The pigments and/or dyes which are used in the paint industry can beused in the base coat compositions of this present invention. Examplesof pigments include metallic pigments, such as aluminium powder andaluminium paste for example; metal based effect pigments, such astitanium oxide, iron oxide and metal powders for example; non-metalliceffect pigments, such as mica for example; carbon black and organicpigments. Metallic pigments which have been pre-treated so as to besuitable for aqueous paints can also be used. The metallic pigments canalso be compounded in combination with the abovementioned non-metallicpigments and dyes.

The compounding proportions of each component are as follows. Thus, inthose cases where an aqueous melamine resin is included, the amountincluded is 1-80% by weight, and preferably 20-60% by weight, withrespect to the solid fraction of the polyurethane dispersion, and theremainder is preferably aqueous polyurethane resin. Moreover, in thosecases where aqueous polyester resin and/or acrylic resin is included,the amounts included are such that the ratio by weight of MelamineResin!: Polyester Resin and/or Acrylic Resin! is 2:1-1:4, and preferably2:1.2-1:2, and the total amount of melamine resin and polyester resinand/or acrylic resin is 1-80% by weight, and preferably 20-60% byweight, with respect to the solid fraction of the polyurethanedispersion, and the remainder is preferably aqueous polyurethane resin.In those cases where a polyether resin is included, the ratio by weightof Melamine Resin!: Resin Selected from among the Group ComprisingPolyester Resin, Acrylic Resin and Polyether Resin (referred tohereinafter as "polyester resin etc.")! is 2:1-1:4, and preferably 2:1.2-1:2, and the total content of melamine resin and polyester resin etc.is 1-80% by weight, and preferably 20-60% by weight, with respect to thesolid fraction of the polyurethane dispersion, and the remainder ispreferably aqueous polyurethane resin.

In those cases where a rheology control agent is included, the amountincluded is 0-10% by weight, and preferably 0.5-2% by weight, withrespect to the base coat composition, and the remainder is preferablywater, solvent, aqueous polyurethane resin, melamine resin or polyesterresin etc. for example. The amount of rheology control agent includedwith respect to the solid fraction of the base coat composition ispreferably 0-20% by weight, and most desirably 2-80% by weight. That isto say, in those cases where no melamine resin or polyester resin etc.is included in the solid fraction, the amount of aqueous polyurethaneresin included is preferably 80-100% by weight, and most desirably90-98% by weight. In those cases where melamine resin and polyesterresin etc. are included, the amount of aqueous polyurethane resinincluded is preferably 5-90% by weight, and most desirably 30-80% byweight, and the total amount of melamine resin and polyester resin etc.included is preferably 1-80% by weight, and most desirably 20-70% byweight.

The amount of metallic pigment and/or effect pigment included ispreferably 0.5-25% by weight, and most desirably 3-24% by weight, withrespect to the solid fraction of the binder.

In those cases where the compounding proportions of each of theabovementioned components are combined within the preferred ranges inall cases, it is possible to obtain a base coat composition which hasexcellent storage stability, with which paint films which have anexcellent finished appearance and paint film performance can be formed,and with which the balance of these properties is very good, and if theamounts of certain components compounded are in the preferred range thena constant effect can be obtained even in those cases where theremaining components are compounded in amounts wider than the preferredranges.

The base coat compositions of this present invention are aqueous basecoat compositions comprising an aqueous dispersion in which the variouscomponents described so far are dispersed in a water based medium.

The solid fraction concentration of the base coat composition ispreferably 15-50% by weight at the painting viscosity. The solidfraction concentration affects the base coat paint colour, and goodpaint colours are obtained with the abovementioned concentrations, butin the case of a base coat composition which has a metallic colour, asolid fraction concentration of 17-25% by weight is preferred since thisgives an especially good paint colour, and in the case of a solid colouran especially good paint colour is obtained with a solid fractionconcentration of 30-45% by weight and so this is preferred.

Water, or a medium in which a small amount of organic solvent isincluded in water, or the reaction liquid in which the aqueouspolyurethane resin has been polymerized, for example, can be cited forthe abovementioned water based medium. It is desirable that the amountof organic solvent included should be as small as possible, and it ispreferably not more than 15% by weight for example.

The base coat compositions of this present invention can be obtained bydispersing the polyurethane resin etc. in the water based medium withthe addition of a primary--tertiary organic amine, such astriethylamine, diethylethanolamine or N-methylmorpholine for example, orammonia. In this case, methods in which the aqueous polyurethane resinis dispersed in the way mentioned above and other essential componentsare added to this polyurethane dispersion, for example, can be adopted.The pH of the aqueous dispersion obtained is 6-9, preferably 7.5-8.5,and most desirably 8-8.4.

Ultraviolet absorbers, photostabilizers, levelling agents, anti-foamingagents, plasticizers, pigment dispersants and hardening catalysts, forexample, can be compounded in a base coat composition of this presentinvention, as required, within the ranges where they do not impede thepurpose of the invention.

No particular limitation is imposed upon the material on which a basecoat composition of this present invention is to be painted, and thematerial may be, for example, metal, plastic, wood or paper. From amongthese materials, metals, and especially steel sheet for automobilepurposes, and plastics are preferred.

The base coat compositions of this present invention can be used notonly for initial painting of an object which is to be painted asdescribed above but also for repainting purposes.

Any solvent type, aqueous or powder transparent clear coat can be usedfor the top coat which is painted over a base coat composition of thispresent invention. These transparent clear coats may be clear coatswhich do not contain any pigment at all, or they may be transparentclear coats which contain a small amount of pigment.

The formation of the paint film is preferably the aforementioned methodof forming a multi-layer paint film, with the clear coat being paintedon with a so-called wet-on-wet procedure, but it is not limited to thismethod. For example, a multi-layer paint film can be formed by paintinga base coat composition of this invention onto the aforementioned objectwhich is to be painted, and forming a paint film of the base coatcomposition by drying for a short time at room temperature to about 80°C., for example for 5 minutes at 80° C., painting the aforementioned topcoat onto this paint film without baking, and then baking the base coatand the top coat at the same time. The baking conditions differaccording to the type of clear coat, but generally conditions of atemperature of 75°-170° C. and a time of 15-30 minutes are preferred.The film thickness of the dry base coat paint film is preferably 7-30mm.

EFFECT OF THE INVENTION

It is possible to form multi-layer paint films which have a goodfinished appearance with the method of forming a multi-layer paint filmof this present invention. In particular, it is possible to formmulti-layer paint films which have an especially good finishedappearance by using a base coat composition which contains the specifiedamount of a specified rheology control agent.

The base coat compositions of this present invention have excellentstorage stability and can form paint films which have a good finishedappearance. In particular, the base coat compositions which contain aspecified amount of a specified rheology control agent have especiallygood storage stability and can form paint films which have an especiallygood finished appearance. Consequently, the base coat compositions ofthis present invention can be used advantageously with steel sheets orplastics, and especially with automobiles.

ILLUSTRATIVE EXAMPLES

Illustrative examples of the invention are described below. In theseexamples the term "parts" signifies "parts by weight" and the term "%"signifies "% by weight".

Example of Manufacture 1 (Manufacture of a Polyurethane Dispersion)

Polymer fatty acid (798 g) this compound was a dimerized fatty acid,(trade name PREPOL 1009, manufactured by the Unichem International Co.)prepared from 1,6-hexanediol and maleic anhydride, average molecularweight 1400!, 12.5 g of 1,6-hexanediol, 65 g of dimethylolpropionic acidand 529.5 g of methyl ethyl ketone were introduced into a four-neckedflask which had been furnished with a stirrer, a thermometer, a refluxcondenser and a nitrogen gas delivery tube and, after mixing, 329 g of1,3-bis(2-isocyanatopropyl-2-yl)benzene m-TMXDI, manufactured by theAmerican Cyanamid Co.! were added.

Reaction was continued by maintaining at 82° C. with stirring and mixinguntil the free isocyanate group content reached 1% of the total amountintroduced. Then, 31 g of trimethylolpropane were added and the mixturewas maintained at 82° C. The rise in viscosity of the reaction mixturewas checked and the reaction was continued until it reached 5 dpa*s(this required about 5 hours). The viscosity was measured at 23° C. bydissolving a 10 ml sample in 10 ml of N-methylpyrrolidone and then usinga Corn Plate type viscometer.

Next, 54 g of butanol were added and the reaction was stopped. Moreover,a mixture of 38 g of dimethylethanolamine and 3254 g of deionized waterwas added and the mixture was stirred for about 2 hours, after which themethyl ethyl ketone was removed by distillation under reduced pressure.A polyurethane dispersion of solid fraction 31% was obtained in thisway.

Example of Manufacture 2 (Manufacture of a Polyester Resin)

Neopentyl glycol (729%), 827% of hexanediol, 462% of hexahydrophthalicacid anhydride and 1710% of polymer fatty acid (dimer content at least98%, trimer content at most 2%, trace monomer content) were introducedinto a four necked flask which had been furnished with a water separatorand the temperature was raised while stirring. At this time, care wastaken to ensure that the temperature of the column attached to the waterseparator did not exceed 100° C. An esterification reaction was carriedout at a maximum temperature of 220° C. and this was continued until theacid value reached 8.5. After cooling to 180° C., 768% of trimelliticacid anhydride was added and the esterification reaction was continueduntil the acid value reached 30.

Next, the reaction mixture was cooled to 120° C. and 1410% of butanolwas added and dissolved. Moreover, after cooling to 90° C., 16.2% ofdimethylethanolamine was added and then 1248% of deionized water wasadded. A polyester dispersion of pH 7.8, solid fraction 60% and acidvalue 30 was obtained in this way.

ILLUSTRATIVE EXAMPLE 1

1) Preparation of an Aqueous Base Coat Composition

An aqueous base coat composition was prepared in the following way inaccordance with the example of manufacture disclosed in German PatentDE-OS-4107136.

First of all, 2.7% of butylcellosolve and 6.4% of commercial melamineresin solution (CYMEL 327, trade name, manufactured by the AmericanCyanamid Co., 90% isobutanol solution) were added with stirring to 34.3%of rheology control agent solution and dissolved. Next, 35.7% of thepolyurethane dispersion obtained in Example of Manufacture 1 was addedwith stirring and a melamine resin--polyurethane resin mixture wasobtained. A 3% aqueous solution of sodium magnesium silicate (LAPONITERD, trade name, manufactured by the Laport Co.) was used for theabovementioned rheology control agent. The electrical conductivity ofthe rheology control agent aqueous solution was 886 mS/cm. A TOA CM-20Selectrical conductivity meter (trade name, manufactured by the Toa DenpaKogyo Co.) was used to measure the electrical conductivity.

A slurry of aluminium pigment was prepared in the following way inparallel with the procedure described above. A stable commercialaluminium paste (average particle size: 15 mm) (3.8%) was uniformlymixed together with 6.0% of butylcellosolve in water. The aforementionedpolyester resin obtained in Example of Manufacture 2 (6.7%) was added tothis mixture and stirred, and a slurry of aluminium pigment wasobtained.

This slurry of aluminium pigment was added to the aforementionedmelamine resin--polyurethane resin mixture and stirred. Finally, about4.4% of deionized water was added, the pH was adjusted to the range8.10-8.30 with dimethylethanolamine solution (10% deionized watersolution) and an aqueous base coat composition was obtained.

2) Painting Tests

The aqueous base coat composition obtained in 1) above was adjusted to asolid fraction concentration of about 24.0% using deionized water insuch a way as to provide a viscosity with which spray painting waspossible. Next, a commercial electrodeposition paint and a mid-coatpaint were painted onto a steel sheet which had been subjected to aphosphoric acid chemical forming treatment and a test sheet wasobtained, and the aqueous base coat composition was painted onto thetest sheet using an air spraying type painting machine in such a way asto provide a dry film thickness of 13-16 μm.

Next, this was dried for 10 minutes at 80° C. in a drying oven. Next,after painting with a commercial one-liquid type clear coat (acrylicresin/melamine resin type), it was baked for 25 minutes at 140° C. in adrying oven. The dried paint film thickness of the top coat clear was 40μm. The finished appearance etc. of the paint film obtained weremeasured. The results are shown in Table 1.

3) Paint Storage Stability at High Temperature Tests

The aqueous base coat composition obtained in 1) above was stored for 1month at 40° C. Subsequently, painting was carried out in the same wayas in 2) above and a paint film was formed. The results are shown inTable 2.

ILLUSTRATIVE EXAMPLE 2

The same procedure as in Illustrative Example 1 was carried out exceptthat a 3% aqueous solution of sodium magnesium silicate (Laponite RD,trade name, manufactured by the Laport Co.) which had been washed withwater in such a way that the electrical conductivity in a 3% aqueoussolution was 760 μS/cm was used for the rheology control agent solutionin Illustrative Example 1. The results are shown in Table 1 and Table 2.

ILLUSTRATIVE EXAMPLE 3

The same procedure as in Illustrative Example 1 was carried out exceptthat a 3% aqueous solution of sodium magnesium silicate (LAPONITE RD,trade name, manufactured by the Laport Co.) which had been washed withwater in such a way that the electrical conductivity in a 3% aqueoussolution was 1034 μS/cm was used for the rheology control agent solutionin Illustrative Example 1. The results are shown in Table 1 and Table 2.

ILLUSTRATIVE EXAMPLE 4

The same procedure as in Illustrative Example 1 was carried out exceptthat a 3% aqueous solution of sodium magnesium silicate (LAPONITE RD,trade name, manufactured by the Laport Co.) which had been washed withwater in such a way that the electrical conductivity in a 3% aqueoussolution was 572 μS/cm was used for the rheology control agent solutionin Illustrative Example 1. The results are shown in Table 1 and Table 2.

                  TABLE 1                                                         ______________________________________                                               Example 1                                                                             Example 2 Example 3 Example 4                                  ______________________________________                                        MF-D Value*1                                                                           75.8      76.8      76.2    62.3                                     IV Value*2                                                                             228       236       231     165                                      Visual   ∘                                                                           ∘                                                                           ∘                                                                         .increment.-x                            Appearance*3                                                                  Run      ∘                                                                           ∘                                                                           ∘                                                                         x                                        Properties*4                                                                  Solid Fraction                                                                            24.0%     24.0%     22.5%                                                                                 30.5%                                 Viscosity                                                                              28-       25-       40-     20-                                      (DIN 4 cup)                                                                   ______________________________________                                         Notes on Table 1                                                              *1 MFD Value: The gloss at was measured at angles of 25° and           70° using a goniophotometer GP2 (manufactured by the Carl Zeiss        Co.), and the MFD value was calculated using the formula indicated below.     MFD Value = {(L25°-L70°)/L70°} × 50                The MFD value is a standard value for assessing the metallic effect of a      base coat/clear coat paint film, and a higher numerical value indicates a     better metallic effect.                                                       *2 IV Value: This was measured using an LMR100 (trade name, manufactured      by the Kansai Paint Co.). A higher numerical value indicates stronger         metallic effect and a better finished appearance.                             *3 Visual Appearance: The appearance of the paint film was evaluated          according to the following standards:                                         x: Poor                                                                       .increment.: Rather poor                                                      ∘: Excellent                                                      *4 Run Properties:                                                            The extent of running (flow) on painting the base coat on a vertical part     was evaluated according to the following standards:                           x: Poor                                                                       .increment.: Rather poor                                                      ∘: Excellent                                                 

As is clear from the results shown in Table 1, the run properties werepoor in Illustrative Example 4 and so a poor result was also obtainedfor the finished appearance. This was because a sodium magnesiumsilicate which had too low an electrical conductivity had been used forthe rheology control agent. The paste viscosity of the rheology controlagent falls as the electrical conductivity becomes lower. Consequently,on adjusting to a viscosity which is suitable for spraying, the solidfraction is very high, and the distribution properties of the aluminiumpigment (the metallic appearance) become poor. The painted appearance inIllustrative Examples 1 and 2 was excellent.

                  TABLE 2                                                         ______________________________________                                                   Example                                                                              Example  Example  Example                                              1      2        3        4                                         ______________________________________                                        MF-D Value                                                                             Before  75.8     76.8   75.2   62.3                                  *1       Storage                                                                       After   56.7     76.4   35.9   51.4                                           Storage                                                              IV Value Before  228      236    231    165                                   *2       Storage                                                                       After   190      224    124    150                                            Storage                                                              Visual           .increment.                                                                            ∘                                                                        x      ∘-x                       Appearance*3                                                                  Run              .increment.-x                                                                          ∘                                                                        .increment.                                                                          x                                     Properties*4                                                                  Solid Fraction      24.0%    24.0%                                                                                24.0%                                                                                31.0%                              After Storage                                                                 Viscosity After  20-.sup. 26-.sup.                                                                             25-.sup.                                                                             20-.sup.                              Storage                                                                       (DIN 4 cup)                                                                   ______________________________________                                         *1-*4: See Table 1                                                       

As is clear from Table 2, the painted appearance after storage was notgood in Illustrative Example 3. This is because coagulation occurredduring the storage of the paint. Furthermore the viscosity under highshear and low shear after storage was markedly reduced. Consequentlyruns were produced on painting. The change in the viscosity afterstorage was small in Illustrative Example 4 but the painted appearancewas poor after storage in the same way as before storage.

On the other hand, in Illustrative Example 2 a good finished appearancewas seen after storage in the same way as before storage. Furthermore,there was virtually no change in the viscosity under high shear andunder low shear. In Illustrative Example 1 there was a small change inthe viscosity on storage and so the painted appearance was rather poorwhen compared with that in Illustrative Example 2, but it was good whencompared with that in Illustrative Example 3 or Illustrative Example 4.

We claim:
 1. A method of forming multi-layer paint films wherein amulti-layer paint film is formed on a substrate comprising:A) applyingto the substrate a base coat composition which is aqueous and whichcontains pigment; B) forming a paint film of the composition which hasbeen applied in A); C) applying a transparent top coat over the basecoat paint film of (B); and D) baking the base coat and the top coat atthe same time;wherein the base coat composition comprises (a.) anaqueous polyurethane resin having a solid fraction, wherein the aqueouspolyurethane resin has structural units represented by the generalformula (1) indicated below and is dispersed in water: ##STR5## in thisformula, Y represents an aromatic, aliphatic or alicyclic hydrocarbylgroup and (b.) an inorganic rheology control agent of which a 3% byweight aqueous solution exhibits an electrical conductivity of 700-900μS/cm.
 2. The method of claim 1 wherein the base coat compositionapplied further comprises from 1-80% by weight of an aqueous melamineresin based on the weight of the solid fraction of the polyurethaneresin.
 3. The method of claim 2 wherein the base coat compositionapplied comprises a binder selected from the group consisting ofpolyester resin, acrylic resin and mixtures thereof, and the ratio byweight of aqueous melamine resin to binder is 2:1-1:4, and the totalamount of melamine resin, polyester resin and/or acrylic resin is 1-80%by weight based on the total weight of the solid fraction of thepolyurethane resin.
 4. The method of claim 1 wherein the base coatcomposition applied further comprises the inorganic rheology controlagent in an amount of from 0.5 to 10% by weight, based on total weightof the base coat composition.
 5. The method of claim 4 wherein the basecoat composition applied comprises the inorganic rheology control agentin an amount of 0.5-2% by weight, with respect to the total weight ofthe base coat composition.
 6. The method of claim 1 wherein a 3% aqueoussolution of the inorganic rheology control agent exhibits an electricalconductivity of 700-780 μS/cm.
 7. The method of claim 4 wherein the basecoat composition applied comprises sodium magnesium silicate as theinorganic rheology control agent.
 8. The method of claim 1 wherein thebase coat composition further comprises 0.5-25% by weight of pigmentselected from the group consisting of metallic pigments, effect pigmentsand mixtures thereof.
 9. The method of claim 1 wherein the base coatcomposition has a pH value of between 6 and
 9. 10. The method of claim 9wherein the base coat composition has a pH value of between 7.5 and 8.5.11. The method of claim 1 wherein the multi-layer paint film is formedon a plastic substrate.
 12. A basecoat composition comprising(a.) anaqueous polyurethane resin having a solid fraction, wherein the aqueouspolyurethane resin has structural units represented by the generalformula (1) indicated below and is dispersed in water: ##STR6## in thisformula, Y represents an aromatic, aliphatic or alicyclic hydrocarbylgroup, and (b.) an inorganic rheology control agent of which a 3% byweight aqueous solution exhibits an electrical conductivity of 700-900μS/cm.
 13. The base coat composition of claim 12 further comprising from1-80% by weight of an aqueous melamine resin based on the weight of thesolid fraction of the polyurethane resin.
 14. The base coat compositionof claim 13 comprising a binder selected from the group consisting ofpolyester resin, acrylic resin and mixtures thereof, and the ratio byweight of the aqueous melamine resin to binder 2:1-1:4, and the totalamount of aqueous melamine resin, polyester resin and/or acrylic resinis 1-80% by weight based on the total weight of the solid fraction ofthe polyurethane resin.
 15. The base coat composition of claim 12further comprising the inorganic rheology control agent in an amount offrom 0.5 to 10% by weight, based on total weight of the base coatcomposition.
 16. The base coat composition of claim 15 wherein theinorganic rheology control agent is present in an amount of 0.5-2% byweight, with respect to the total weight of the base coat composition.17. The base coat composition of claim 12 wherein a 3% aqueous solutionof the inorganic rheology control agent exhibits an electricalconductivity of 700-780 μS/cm.
 18. The base coat composition of claim 12wherein the inorganic rheology control agent comprises sodium magnesiumsilicate.
 19. The base coat composition of claim 12 further comprising0.5-25% by weight of pigment selected from the group consisting ofmetallic pigments, effect pigments and mixtures thereof.
 20. The basecoat composition of claim 12 wherein the composition has a pH value ofbetween 6 and
 9. 21. The base coat composition of claim 12 wherein thecomposition has a pH value of between 7.5 and 8.5.
 22. The method ofclaim 1 wherein the transparent top coat is an aqueous top coat or apowder top coat.